Here is an update on my 3x4 project. I am modifying the back to fit 4x5 film holders. Essentially I have done three things; I reset the side rails to accommodate the 4x5 holders, I cut a new key slot and removed the end block so that a 4x5 holder will come just to the edge of the back(this puts the new key slot 3/16" from the edge), and I extended the edges of the spring back itself to fit the new slot. Removing the stop block at the end caused some light leak problems and so I have added (for now anyway) a piece of weather strip)

The polaroid 545 backs will fit a standard 4x5 springback, the 405 back however is too thick. You can shim out the springs using longer 4/40 screws, which I did for awhile, but I found that the springs did not hold a standard cut film holder very tightly after I did that. Recently I made the spring extensions shown in the pics. The construction should be fairly obvious from the picture, the spring material was cut from the blade of a spackling trowel (buy the ones that say "flexible blue steel")These extensions work very well on both my 4x5 and on the modified 3x4, they will extend to hold a 405 back yet still keep a standard cutfilm holder tight.